SAE TECHNICAL
PAPER SERIES
952097
Comparison of Two and Four Track
Machines to Rubber Tire Tractors
in Prairie Soil Conditions
Reed J. Turner
Alberta Farm Machinery Research Centre
The Engineering Society
For Advancing Mobility
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I N T E R N A T I O N A L
International Off-Highway &
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Milwaukee, Wisconsin
September 11-13, 1995
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel:(412)776-4841 Fax:(412)776-5760
952097
Comparison of Two and Four Track Machines to
Rubber Tire Tractors in Prairie Soil Conditions
Reed J. Turner
Alberta Farm Machinery Research Centre
The tests were a joint effort of AFMRC in Lethbridge,
Alberta and the Northern Tractor Resource Center
(NTRC) in Havre, Montana to provide information about
the power delivery characteristics of each of the traction
systems. Case-IH, Caterpillar, Firestone, Flexi-coil, Ford
New Holland, Gilbert and Riplo, Goodyear, John Deere,
Leon Manufacturing, Morris Industries, and numerous
farmers cooperated in the tests.
ABSTRACT
Traction performance tests were run by Alberta
Farm Machinery Research Centre personnel on fourwheel drive tractors with single, dual and triple rubber
tires and on track tractors with two and four rubber belt
tracks. Traction power delivery efficiency was compared
in various tilled and untilled soil conditions representative
of the Northern Great Plains. Various ballast set-ups and
tire inflation pressures were tested. Power hop problems,
steering performance, ease of set-up and overall system
costs were also evaluated.
LITERATURE REVIEW AND DISCUSSION
Several researchers have reported the differences in
traction performance between single and dual tires,
between single tires and tracks, and between dual tires
and tracks.
Domier et al., 1970 (1), compared a dual 18.4-38
bias-ply two-wheel drive, a dual 18.4-38 bias-ply fourwheel drive and a steel tracked tractor. They reported an
8 percent increase in tractive efficiency for the tracks over
the four-wheel drive.
Taylor and Burt, 1975 (2), compared a single bias
12.4-28 tire at an inflation pressure of 98 kPa (14 psi)
and loads of 500 kg (1,100 lb) and 1,000 kg (2,200 lb)
to a steel track. They found tractive efficiencies for the
track to be 20 percent higher than for the tire.
Esch et al., 1986 (3), compared rubber belt tracks to
a four-wheel drive equipped with dual 20.8-38 bias-ply
tires. They reported a 7 to 12 percent increase in tractive
efficiency for the tracks.
Evans and Gove, 1986 (4), compared a dual 20.8
R38 radial-ply four-wheel drive and a rubber belt tracked
tractor. They reported a 20 percent increase in tractive
efficiency for the tracks in loose soil and a 6 percent
increase in firm soil.
Bashford et al., 1987 (5), compared single and dual
18.4-38 bias-ply tires at the same total weight and tire
pressures. They reported that on concrete the singles
had greater tractive efficiency than the duals but there
was no difference in performance between the singles
and duals in the field.
INTRODUCTION
Tractors offer many ways to deliver traction power
to the ground. In the past, the only choices were track
tractors using steel tracks, or two-wheel drive or fourwheel drive tractors using single or dual bias-ply rubber
tires. Today, rubber belt tracks as two parallel tracks or
four articulated tracks, radial-ply tires, triple tires and high
flotation tires have been added to the selection list.
Market conditions and the need for increased efficiency
in agriculture have increased the demand for information
about traction options. Changes in recommended
inflation pressures for radial tires have further complicated
the traction selection and optimization process.
During a four year period, engineers at the Alberta
Farm Machinery Research Centre (AFMRC) tested
various traction systems. Tests were run on single, dual
and triple rubber tires on four-wheel drive tractors.
Different tractor weight set-ups and different tire inflation
pressures were used and included both previous and
current weight and tire inflation recommendations. Tests
were also run on the dual rubber belt track system used
on Caterpillar Challenger (TM) tractors and on the four
rubber belt track system demonstrated on the Case-IH
Quadtrac (TM) prototypes. Tests were conducted in
several soil conditions representative of the Northern
Great Plains.
2
A problem with much of this previous work results
from the tires and tire inflation pressures that were used.
Tires provide better tractive performance as their inflation
pressure is decreased. The performance advantages of
radial tires compared to bias tires and the importance of
setting radial tires to the correct and minimum inflaction
pressure has been documented in several references.
Burt and Bailey, 1981 (6) measured the effect of inflation
pressure and tire load on the tractive efficiency of 20.8
R38 radial tires. They found a decrease in tractive
efficiency as tire inflation pressure increased. Charles,
1984 (7) measured the effect of inflation pressure and tire
load on the tractive efficiency of single radial 18.4 R38
tires. He found a 7 percent decrease in tractive efficiency
with a 55 kPa (8 psi) increase in inflation pressure at a
tire load of 2,700 kg (5,940 lb) per tire. Wulfsohn et al.,
1988 (8) compared the performance of a number of radial
and bias tires and found an average increase in tractive
efficiency of 6 to 8 percent for radial tires across the 0 to
30 percent slip range. Bashford et al., 1992 (9),
compared performance of three different radial tires at
three inflation pressures and reported lower tractive
efficiencies at the higher pressures.
The minimum allowable inflation pressure for a tire is
a function of the weight the tire carries. Tire
manufacturers publish load-inflation tables that list the
correct inflation pressure for various loads. In late 1991
to early 1992, the load-inflation tables for radial tires were
revised to allow lower inflation pressures than what had
been recommended in the past. Tire manufacturers
suggest that following these new guidelines will result in
improved tractive performance with radial tires, Goodyear,
1992 (10). Many of the existing traction performance
tests were run with bias tires, often operated at pressures
that exceeded even the levels recommended at the time.
While the reported data is probably correct, the
overpressures and limited use of radial tires reduce the
current value of such tests. Considering the five abovementioned tire and track comparisons, the following can
be observed.
For Domier et al., 1970 (1), the four-wheel drive tires
were inflated to 110 kPa (16 psi) inner and 96 kPa (14
psi) outer with a load of 1,480 kg (3,260 lb) front and
1,160 kg (2,550 lb) rear per tire. The correct inflation
pressure for these tire loads would have been 82 kPa (12
psi) for all the tires. This means the tests were run at a
nominal 20 kpa (3 psi) overpressure. Had the tires been
radials, the 1992 correct inflation pressure would have
been 62 kPa (9 psi) front and 41 kPa (6 psi) rear.
For Taylor and Burt, 1975 (2), the correct inflation
pressure for the tire loads would have been 82 kPa
(12 psi) at an 850 kg (1,870 lb) load and 110 kPa (16 psi)
at the 1,000 kg (2,200 lb) load. At the 500 kg (1,100
lb) load the tire was significantly over inflated. The data
shows that the 1,000 kg (2,200 lb) load had higher
tractive efficiency than the 500 kg (1,100 lb) load.
For Esch et al. (3), the tractor tires were inflated to 95
kPa (14 psi) inner and 85 kPa (12 psi) outer with a load
of 1,900 kg (4,180 lb) front and 1,370 kg (3,010 lb) rear
per tire. The correct inflation pressures for the tire loads
would have been 82 kPa (12 psi) for all the tires. While
this was close to what was actually run, had the tires
been radials, the 1992 correct pressures would have
been 69 kPa (10 psi) front and 41 kPa (6 psi) rear.
Evans and Gove, 1986 (4), used four-wheel drive tires
that were inflated to 83 kPa (12 psi) inner and outer at a
load of 1,800 kg (3,960 lb) front and 1,500 kg (3,300 lb)
rear per tire. These were correct inflation pressures for
the time. The 1992 correct inflation pressures for these
tires and loads would have been 62 kPa (9 psi) front and
48 kPa (6 psi) rear, a 20 kPa (3 psi) front and 41 kPa (6
psi) rear reduction in pressure from what was tested.
For Bashford et al., 1987 (5), the tires were inflated to
a pressure of 124 kPa (18 psi) at a load of 2,300 kg
(5,060 lb) for each single and 1,150 kg (2,530 lb) for
each dual. The correct inflation pressure for the singles
at the 2,300 kg (5,060 Ib) load would be 110 kPa (16 psi),
and for the duals at the 1,150 kg (2,530 lb) load, 82 kPa
(12 psi). For this tractor, the duals were significantly
more over inflated, 41 kPa (6 psi), than the singles, 13
kPa (2 psi). Had the tires been radials, the 1992 correct
inflation pressure for the singles at the 2,300 kg (5,060
lb) load would have been the same 110 kpa (16 psi),
while for the duals at the 1,150 kg (2,530 lb) load the
pressure would have been 48 kPa (6 psi).
In each of these cases, using correct inflation
pressures would have changed the results. Using radial
tires with correct inflation pressures by the 1992
standards would have changed the results even more.
SCOPE OF THE TESTS
These tests were to provide information to help
farmers in the selection and optimization of appropriate
traction systems. Tests were run from 1991 to 1994 in
conditions representative of the Northern Great Plains.
All the tractors that were used were instrumented and
data was taken over a range of loads and speeds.
Engineers at AFMRC previously developed a
performance measurement system and procedure to
simplify traction performance measurements (11). This
set-up was used throughout the tests and included an onboard data acquisition system that was portable between
vehicles (12). The test equipment was powered by the
vehicle and operated by the vehicle driver. Additional
information about the measurements, the test procedures
and the specific test sites is in the Appendix.
In southern Alberta in the summer of 1991, a Case-IH
9260, a Case-IH 9250 and a Caterpillar Challenger 65
with 610 mm (24 in) wide rubber belt tracks were tested
in primary and secondary tillage in a clay-loam, a sandy
loam and a heavy clay soil. Both rubber tire tractors were
tested with dual radial tires. A single inflation pressure,
ballast weight and ballast ratio was maintained using fluid
ballast.
In northern Montana in the fall of 1991, a John Deere
8760, a John Deere 8960, a Caterpillar Challenger 75
with 700 mm (27.5 in) wide tracks and a Caterpillar
3
The test tractors were at manufacturers' rated engine
power levels that ranged from 201 to 276 kw (270 to 370
hp). Measured engine power levels were typically 5 to 15
percent above the advertised values. To remove the
effects of engine rating differences, overpowering and
engine power variability, and because traction
performance is ultimately determined by the amount of
available power that can be transferred to the ground,
traction performance results were compared using power
delivery efficiency. Power delivery efficiency is defined as
the ratio of the power entering a traction system
compared to the power delivered to the ground. This
basis for comparison kept the capabilities of the traction
power delivery systems independent of the associated
engine systems. Testing also determined the settings and
conditions where power delivery was maximized and
evaluated how the various traction systems functioned.
Factors such as power hop, steering, ease of set-up and
system cost were considered.
The tests were intended to determine the effect of the
traction systems on farm operations from a practical point
of view. Data from a test was evaluated without post
processing and with minimum refinement, and as much
as possible, from the viewpoint of a farmer. After a run,
plots were made of pull-to-weight ratio versus slip,
drawbar power versus slip, percent power delivered
versus slip, and percent power delivered versus pull-toweight ratio. Figure 1, A,B C and D, is a set of these
plots. Maximum values, their location and the general
shape of the curves were recorded. Additional notes
were recorded about ride roughness, power hop if it
occurred, and travel speed required to obtain maximum
drawbar power.
Challenger 75 with 890 mm (35 in) wide tracks were
tested in primary and secondary tillage in a heavy clay
soil. Various combinations of radial and bias single, dual
and triple tires at various tire inflation pressures were
tested. Ballast weights and ratios were varied using both
liquid and cast ballast.
In central Alberta in the early summer of 1992, a John
Deere 8760 and a Caterpillar Challenger 65 were tested
in primary and secondary tillage in a loam soil. The
rubber tire tractor was tested with single, dual and triple
radial tires at various inflation pressures. One ballast
weight and ratio was maintained using cast ballast.
In northern Alberta in the late summer of 1992, a
John Deere 8760 with dual radial tires and a Caterpillar
Challenger 65B were tested in primary and secondary
tillage in a clay-loam soil. Only one tire inflation pressure
and ballast set-up was used.
In southern Alberta in the fall of 1992, a John
Deere 8760 was tested with dual and triple radials in
primary and secondary tillage in a clay-loam soil. Various
tire inflation pressures were tested. One ballast weight
was maintained, but several ballast ratios were tested
using liquid and cast ballast.
In southern Alberta in the summer of 1993, a Ford
New Holland 946 with single, dual and triple radials and
a John Deere 8770 with dual and triple radials were
tested in secondary tillage in a clay-loam soil. Various
tire inflation pressures and ballast weights and ratios were
tested using liquid and cast ballast.
In southern Alberta in the fall of 1994, a Case-IH
9250 equipped with four positive-drive rubber belt tracks
was tested in primary and secondary tillage in a clay-loam
soil. One ballast weight and ratio was maintained. Since
this tractor was unique, a detailed description is included
in the appendix.
Figure 1, A and B
Test Plots from a Typical Test.
A = Pull/Weight versus Slip
B = Drawbar Power versus Slip
4
Figure 1, C and D
Test Plots from a Typical Test.
C = Percent Power Delivered versus Slip
RESULTS AND CONCLUSIONS
D = Percent Power Delivered versus Pull/Weight
than correctly inflated single radial tires. For tire loads
near the lower end of allowable ballast ranges, 50
kg/engine kw (83 lb/engine hp), correctly inflated single
radial tires showed the same efficiency as duals. Figure
2 shows data for 20.8R42 tires for a tractor weight of
16,400 kg (36,000 lbs). This was 4,100 kg (9,000 lb)
per tire for the singles and was just beyond the maximum
allowable for these tires.
The following results and conclusions deal with the
power delivery efficiency of the traction systems. On
some rubber tire tractor configurations, power hop, a
resonant instability producing a fore/aft bouncing or
"porpoising" of a tractor under load, affected performance.
It was usually possible to find a set-up and condition
where a rubber tire tractor configuration showed this
problem. While some information is presented on power
hop because of its influence on the test results, a full
discussion of the causes and cures of power hop is
beyond the scope of this paper.
POWER DELIVERY EFFICIENCY - Tractors with two
rubber belt tracks and those with correctly inflated dual
radial tires showed the highest power delivery efficiency.
For these set-ups, the peak efficiencies were not
significantly different in the conditions tested, although the
peak occurred at a lower slip level for tracks than for
tires, Figure 2.
The effect the number of tires had on power delivery
efficiency depended on the tire loads. Triple radial tires
were lower in efficiency than duals or singles at all tire
loads. For tire loads near the maximum of the allowable
tractor ballast range, 73 kg/engine kw (120 lb/engine
hp), correctly inflated dual radial tires were more efficient
Figure 2. Power Delivery Efficiency at 16,400 kg
(36,000 lb) Total Tractor Weight.
5
tractors. Figure 5 shows a plot of power delivery
efficiency for a 223 kw (300 hp) four-track tractor, a
208kw (270 hp) 2-track tractor and a 223 kw (300 hp)
four-wheel drive tractor, all in the Lethbridge clay-loam
soil. In this particular test, the power delivery efficiency
of the two-track tractor peaked at 78 percent, 1 percent
higher than the 77 percent peak of the four-wheel drive
tractor. The four-track tractor had a peak power delivery
efficiency of 72 percent, 5 percent lower than the
four-wheel drive tractor.
Figure 3 shows data for the same tires as Figure 2
with a tractor weight of 12,300 kg (27,000 lb), or
3,075 kg. (6,750 lb) per tire for singles. The triple tire
efficiency ranged from 83 percent of the dual tire
efficiency, Figure 2, to 94 percent, Figure 3, but other
tests suggested that while triples were always less
efficient than duals, there was no clear trend with
increasing tire loading.
Figure 3. Power Delivery Efficiency for 12,300 kg
(27,000 lb) Total Tractor Weight.
Figure 5. Power Delivery Efficiency - Two Tracks, Four
Tracks and Dual Radial Tires.
Bias-ply tires at their correct inflation pressure had
lower power delivery efficiency than radial tires at their
correct inflation pressures. For 20.8-42 bias-ply dual tires
at a load of 10,100 kg (4,600 lb) and an inflation
pressure of 82 kPa (12 psi), the power delivery efficiency
was 92 percent of that for 20.8 R42 radial dual tires with
the same load and an inflation pressure of 69 kPa
(10 psi), Figure 4.
Rubber belt tracks had a wider range of efficient
operation than even the best rubber tire combinations.
This was true whether considering percent power
delivered versus pull/weight ratio as shown in Figure 6, or
drawbar power delivered versus ground speed as shown
in Figure 7.
Figure 4. Power Delivery Efficiency for Bias
Duals and Radial Duals.
Figure 6. Power Delivery Efficiency versus
Pull-to-Weight at 16,400 kg (36,000 lb)
Total Tractor Weight.
The four-track tractor showed lower power delivery
efficiency than the two-track tractors or the rubber tire
6
Figure 7. Drawbar Power Developed Versus Ground
Speed for a 223 kw (300 hp) Rubber Belt
Tractor and a 223 kw (300 hp) Dual Radial
Tire Tractor.
Figure 9. Effect of Number of Tires on Pull
at Minimum Ballast Weight.
The 210 kw (270 hp) two-track rubber belt tractors
developed maximum drawbar power around 1 to
3 percent slip, while the rubber tire tractors developed
maximum drawbar power around 8 to 12 percent slip.
Figure 10 is a graph showing drawbar power developed
in an 8 km/h (5 mph) gear for singles, duals, triples and
tracks in secondary tillage. The shape is typical for all
the soil conditions tested.
PULL AND TRACTION - Rubber belt tracks developed
higher pull for a given tractor weight and slip level than
any rubber tire combinations. The peak pull produced by
the rubber belts was less affected by soil type or tillage
condition than it was for the rubber tires. The average
pull of the tracks at 10 percent slip for all the conditions
tested was 140 percent of the average pull of the tires in
the same conditions.
For four-wheel drive tractors ballasted to the upper
end of allowable ballast ranges, dual tires increased the
pulVweight ratio at a given slip level when compared to
singles, Figure 8. Moving to triples had no further effect
on pull/weight ratio. At a ballast weight near the lower
end of allowable ballast ranges, moving from singles to
duals to triples had little effect on the pull/weight ratio,
Figure 9.
Figure 10. Drawbar Power Developed Versus
Slip for Tires and Tracks.
As engine power increased on the two-track rubber
belt tractors, their ability to deliver full drawbar power at
low speeds and slip was reduced and they responded
more like rubber tire tractors. The Caterpillar Challenger
65, 65B and 75 had rated engine power levels of 201,
212 and 242 kw (270, 285 and 325 hp), respectively.
Since they all weighed about 14,500 kg (31,900 lb), the
weight-to-power ratios were respectively 72, 68 and 60
kw/kg (118, 112 and 98 lb/hp). Figure 11 is a graph of
drawbar power developed in a 6.3 km/h (4 mph) gear for
each of these tractors and shows a more gradual slip
Figure 8. Effect of Number of Tires on Pull at
Maximum Allowable Ballast Weight.
7
proportion to the weight added. There was no effect on
power delivery efficiency or pull-to-weight ratio. At the
time of these tests, there were no ballast options for the
rubber belt tractors so their weight was not changed. The
effect of reducing the weight-to-power ratio by increasing
the power was discussed previously.
The weight-to-power ratio of rubber tire tractors
affected power hop. Heavier tractors experienced less
problems with power hop than lighter tractors of the same
power level. Heavy tractors were less likely to hop, and
when they did, the hop was less severe than it was for
lighter tractors. This could have been because of the
additional mass or because the heavier tractors operated
at lower slip levels for the same load.
curve for the 65B and the 75 at maximum drawbar power.
In an 8 km/h (5 mph) gear, only the 75 had a gradual slip
curve, and in a 10 km/h gear (6 mph) all the tractors had
a steep curve like the 65 shown in Figure 11.
BALLAST RATIO EFFECTS (rubber tire tractors only)
- Changing the front/rear static ballasted weight ratio from
60/40 to 55/45 to 50/50 had no effect on overall power
delivery efficiency or on peak pull. It did affect the power
delivered by the front axle and the rear axle. As shown
in Figure 13, for radial duals at a pull/weight ratio of 0.35,
the 60/40, 55/45 and 50/50 ratio tractors had front-to-total
power ratios of 0.52, 0.46 and 0.40. Single and triple
tires showed similar ratios. These front-to-rear power
ratios were measured while pulling a floating hitch
cultivator and would be further reduced when pulling an
implement with a downward hitch load.
Figure 11. Drawbar Power Developed versus
Slip for Cat 65, 65B and 75.
On the four-track rubber belt tractor, the weight was
16,700 kg (36,850 lb) and the power was 223 kw
(300 hp). These were not varied, but the shape of the
drawbar power versus slip curve was similar to one for
the 208 kw (270 hp) two-track Challenger 65, Figure 12.
Both these tractors had a weight-to-power ratio around
73 kg/kw (120 lb/hp).
Figure 13. Effect of Front/Rear Weight Ratio on
Front/Rear Power Ratio.
Power hop occurred more frequently and was more
difficult to control on tractors at 50/50 ballast ratio than on
tractors at 55/45 ratio. This could have been because
both ends of the tractor had equivalent stiffness (spring
rate) at the 50/50 ratio.
Figure 12. Drawbar Power versus Percent Slip for
Two and Four Track Tractors.
The two-track and four-track rubber belt systems
tested had adequate traction to deliver 150 kw (200 hp)
at the drawbar at field speeds of 5km/h (3.1 mph) and
above in the soil conditions tested. All the rubber tire
power delivery systems tested had adequate traction to
deliver 150 kw (200 hp) at field speeds of 8 km/h (5 mph)
and above.
TYPE OF BALLAST EFFECTS (rubber tire tractors
only) - The type of ballast, whether cast mounted on the
axle or on the frame, or fluid in the tires, had no effect on
power delivery efficiency or peak pull.
Some tractor and tire combinations with cast ballast
at the rear were somewhat less prone to experience hop
than those with fluid ballast at the rear. Other
BALLAST EFFECTS - Adding ballast to a tractor
increased pull at a given slip and total pull in direct
8
would have been decreased by raising pressures in the
tires on both ends. Figure 15 is a graph showing how the
power delivery efficiency was reduced across the range
of pull/weight as the tractor tires were over inflated, first
on one end, then on the other, and then on both. For the
front tires the rated pressure was 62 kPa (9 psi) and the
over inflation or high pressure was 124 kPa (18 psi). For
the rear, the rated pressure was 48 kPa (7 psi) and the
high pressure was 96 kPa (14 psi). In this particular test
the tractor experienced power hop at both the rated
front/rated rear pressures and the high front/high rear
pressures. The hop was removed with both the high
front/rated rear and the rated front/high rear pressures.
combinations with fluid at the rear were much less prone
to hop than the same combinations with cast at the rear.
When hop did occur, it was easier to control on tractors
with fluid ballast than it was on tractors with cast ballast.
The fluid was typically concentrated at the rear of the
tractor, where it increased the stiffness of the tires on the
rear. Different stiffness front to rear was important in
controlling power hop.
TIRE INFLATION PRESSURE EFFECTS (rubber tire
tractors only) - Tire inflation pressure had a significant
effect on the performance of radial tire equipped tractors.
The best power delivery efficiency was obtained with tire
pressures set at the 1992 manufacturers' published
minimum inflation pressures for a given tire load. As
shown in Figure 14, for dual tires, increasing the inflation
pressures from the correct 55 kPa (8 psi) for the load to
96 kPa (14 psi) reduced the power delivery efficiency by
7 percent at peak efficiency. Similar reductions occurred
with single and triple radial tires. Bias-ply tires showed a
similar trend but a lower magnitude of effect.
Figure 15. Power Delivered versus Pull-to-Weight
for Various Inflation Pressures.
TRACTOR OPTIMIZATION - Optimizing a rubber tire
tractor for a given ground condition, draft load and speed
was more difficult than it was for a rubber belt tractor.
The rubber belt tractors had no ballast or pressure
adjustments, but still functioned at an optimum over a
wide range of soil conditions and loads. The optimum
range of operation with ballast and inflation pressure set
for a given soil condition and load was more narrow for a
wheel tractor. As discussed previously, both ballast and
tire inflation pressures affected rubber tire tractor
performance. To optimize a rubber tire tractor, both had
to be considered and adjusted. It was possible to
optimize single, dual or triple tires for a given ground.
condition, draft and speed, but the optimum settings and
the optimum performance were not the same for each
configuration.
Figure 14. Effect of Tire Pressure on Power
Delivery Effectiveness.
Tire inflation pressure had a significant effect on
power hop. If hop occurred, it was always possible to
control and/or remove it within the working range of the
tractor by changing tire inflation pressure. The basic
approach to control hop was to soften the tires on one
end of the tractor by reducing their pressure to the
minimum allowable and to stiffen the tires on the other
end of the tractor by increasing their inflation pressure
until the hop disappeared. It was important to increase
the stiffness of the tires on whichever end was already
the stiffer. Typically, this was the front tires on a tractor
with cast ballast, and the rear tires on a tractor with fluid
ballast. There was a trade-off between achieving
maximum tractive efficiency and controlling hop with
inflation pressure. As the inflation pressure was
increased on one end of a tractor, the power delivery
efficiency of the tires on that end was decreased. Raising
pressures on one end decreased the total efficiency for a
tractor by about one third of the amount that the efficiency
STEERING - Steering was more of a problem with
two-track rubber belt tractors than with rubber tire tractors
or four-track rubber belt tractors.
With a light draft load, all the tractors steered well.
Under a heavy draft load the different traction systems
steered differently. The four-track rubber belt tractor
continued to steer just as well as under a light load. The
rubber tire tractors continued tosteer, but showed a
tendency to pull or slip sideways while cornering. The
rubber belt tractors with two tracks steered poorly. Near
9
supplied retail prices from Alberta in the fall of 1992, a
rubber belt tractor with two tracks cost 15 percent more
than an equivalent drawbar horsepower rubber tire tractor
equipped with dual radial tires. Comparing actual prices
paid by farmers in Alberta during the same time, the
difference was even greater, with rubber belt tractors
about 30 percent more than rubber tire tractors. The cost
for a wheel tractor was about the same whether equipped
with single radials or dual radials and about 5 percent
higher when equipped with triples. Based on 1994
figures, the cost of the rubber belt tractor with four tracks
was estimated to be about 35 percent higher than a
comparable rubber tire tractor.
Tractor costs are situation and location dependent,
and can change quickly. The dealer retail prices used for
comparison were the December 1992 retail prices in
Alberta, in Canadian dollars. They included freight,
ballast and delivery to a farm in Alberta, but did not
include any taxes. The actual prices paid by customers
were determined from interviews with customers who
dealt on tractors in Alberta during the November to
December 1992 period, and were in Canadian dollars,
again including freight, ballast and delivery, but not taxes.
their maximum draft they would not steer at all, but would
continue in a straight line when the steering wheel was
turned. Steering returned when the draft was reduced.
The steering response was somewhat improved on a
rubber belt tractor with 890 mm (35 in) wide rubber belts,
but was still inadequate under heavy draft.
Steering the rubber belt tractors with two tracks used
more power than steering the rubber tired tractors or the
rubber belt tractor with four tracks. On the two- track
tractor, the long rubber belt track had a sliding or
scrubbing action as it turned, which used more power
than the rolling action of a tire. Since the rubber belt
differential steering mechanism sped up one track, this
also used additional power.
The rubber tired tractors and the rubber belt tractor
with four tracks steered with little soil disturbance. Any
soil disturbance was proportional to the load and resultant
slip of the tires. The rubber belt tractors with two tracks
disturbed the soil surface when they turned, whether
under load or not. The sliding tracks pushed soil
sideways and produced ridges and depressions.
COMPACTION - Although soil compaction was not
directly measured in the tests, the following compaction
concerns and observations about tires and tracks were
noted.
A common assumption for wheel tractors has been
that average ground pressure can be approximated by
the tire inflation pressure. Given this assumption,
four-wheel drive rubber tire tractors ballasted to over
77,000 kg (35,000 lb) require triple tires to reduce their
average ground pressure to the nominal 41 kPa (6psi)
ground pressure of the two-track rubber belt tractors. As
discussed previously, there is a performance penalty in
using triples to achieve low average ground pressure.
The rubber belt tractor with four tracks had an average
ground pressure of 28 kPa (4 psi). Since tire inflation
pressures are currently limited at the low end to 41 kPa
(6 psi), it would be impossible for rubber tire tractors to
equal this ground pressure.
The theoretical average ground pressure may not be
representative of actual ground pressure for a rubber belt
track under load. Under average to heavy draft load,
weight shifts to the rear of a tractor. On a rubber tire
tractor, the rear tires tend to flatten out as the weight they
carry increases. This increases the contact area and the
average ground pressure remains about the same. On a
rubber belt tractor, there can be no flattening or area
increase, and as weight shifts, the ground pressure at the
rear of the tractor must increase.
In traction overload or a loss of flotation situations, the
rubber tire tractors dug into the ground more rapidly than
rubber belt track tractors, even when both were at similar
average ground pressures. This may have been due to
differences in the shape of the area where the ground
pressure was applied.
TEST PROCEDURE ISSUES - A chisel plow or field
cultivator worked well as an in-the-field dynamometer for
tractor tests and provided satisfactory draft adjustment for
these tests. Best results were obtained with a unit that
was oversized for the tractor being tested. An offset disk
was marginal as a field dynamometer and a deep ripper
was not acceptable because of the difficulty in varying the
draft of these implements.
Percent power delivered, the ratio of drawbar power
to some known engine power level measurement,
measured as described in the Appendix, was a good
substitute for tractive efficiency when comparing tractor
performance.
Average percent slip could be measured accurately
from one specific wheel or track, provided the readings
were averaged for 2 to 4 seconds.
Engine speed was an adequate in-the-field substitute
for engine power level and fuel consumption if the actual
power level and fuel consumption had been measured
and related to engine speed before tests began.
Turbo boost pressure was not an adequate in-the-field
substitute for engine power level with the measurement
equipment used, but might have been acceptable with
more stable pressure gauges.
Fuel rail pressure was not an adequate substitute for
engine power because it remained at an unchanging
peak along the variable speed part of the engine curve.
FUTURE WORK
POWER HOP - As previously noted, power hop had
a significant effect on tractor performance when it
occurred. It was usually possible to make a given tractor
configuration hop with some combination of settings. The
data set from the tests represents a significant resource
COSTS - Rubber belt traction power costs more than
comparable rubber tire traction power. Using dealer10
about power hop causes and controls. Analysis related
to power hop is continuing and will be reported when
complete.
COMPACTION - No direct measurements were taken
on compaction during these tests because of the lack of
wide agreement on a useful compaction parameter that
can be quickly measured. Work is continuing to define a
simple ground compaction parameter or procedure that
can be used effectively at the farm level.
EXTENSION - Work is continuing to put the
information from these tests into an accessible,
understandable and easily-used format to help farmers in
traction decisions, both those associated with buying new
systems and those associated with optimizing existing
traction set-ups.
SUMMARY
Bashford, L.L, K. Yon Bargen, T.R. Way and
L. Xiaoxian, "Performance Comparisons Between
Duals and Singles on the Rear Axle of a FrontWheel Assist Tractor", Transactions of ASAE, Vol.
30(3), pp. 641-645, 1987.
Wulfsohn, D., S.K. Upadhyaya and W.J. Chancellor,
"Tractive Characteristics of Radial-Ply and Bias-Ply
Tyres in a Califomia Soil", Journal of Termmechanics, Vol. 25, No. 2, pp. 111-134, 1988.
9.
Bashford, L.L., S. AI-Hamed and C. Jenane, "Effects
of Tire Size and Pressure on Tractive Performance"
ASAE Paper No. 92-1011, St. Joseph, Michigan,
1992.
14. Turner, R., "Slip Measurement Using Dual Radar
Guns", ASAE Paper No. 93-1031, St. Joseph,
Michigan, 1993b.
APPENDIX
3. Esch, J.H., L.L. Bashford and K. Yon Bargen,
"Tractive Performance of Rubber Belt Track and
Four-Wheel Drive Agricultural Tractors", ASAE Paper
86-1546, St. Joseph, Michigan, 1970.
5.
8.
13. "Uniform Terminology for Traction of Agricultural
Tractors, Self-Propelled Implements, and other
Traction and Transport Devices", 1983-1984
Agricultural Engineers Yearbook of Standards, pp
180, American Society of Agricultural Engineers, St.
Joseph, Michigan, 1983.
Taylor, J.H. and E.C. Burt, "Track and Tire
Performance in Agricultural Soils", Transactions of
ASAE, Vol. 18, pp. 3-6, 1975.
Evans, W.C. and D.S. Gove, "Rubber Belt Track in
Agriculture", ASAE Paper No. 86-1061, St. Joseph,
Michigan, 1986.
Chades, S.M., "Effects of Ballast and Inflation
Pressure on Tractor Tire Performance", Agricultural
Engineering, Vol. 65, No. 2, pp. 11-13, 1984.
12. Turner, R., "Instrumentation for In-Field Agricultural
Machinery Testing", ASAE Paper No. 92-118PNW,
St. Joseph, Michigan, 1992.
Domier, K.W., O.H. Friesen and J.S. Townsend,
"Traction Characteristics of Two-Wheel Drive, FourWheel Drive and Crawler Tractors", ASAE Paper
No. 70-149, St. Joseph, Michigan, 1970.
4.
7.
11. Turner, R., "A Simple System for Determining
Tractive Performance in the Field", ASAE Paper No.
93-1574, St. Joseph, Michigan, 1993a.
REFERENCES
2.
Burt, E.C. and A.C. Bailey, "Interaction of Dynamic
Load and Inflation Pressure on Tire Performance",
ASAE Paper No. 81-1537, St. Joseph, Michigan,
1981.
10. Optimum Tractor Tire Performance Handbook, The
Goodyear Tire and Rubber Company, Akron, Ohio,
January 1992.
Traction power is an important part of an overall farm
efficiency and cost control strategy. While there is a
great deal of information available about traction systems,
some of the information is of less value today because it
no longer reflects current understanding and standards.
The Alberta Farm Machinery Research Centre has
developed a current package of efficiency and
performance information about traction systems. This
information can be used to assist farmers in making the
best use of their existing tractors and can help them to
select the most appropriate traction delivery systems for
their operation.
1.
6.
DETAILED TEST INFORMATION
Test Instrumentation and Computations - The
terminology used conforms wherever possible to ASAE's
standard traction and tractor terminology (13). The
values measured and the methods of measuring them
were as follows:
Draft - the horizontal pull a tractor developed was
measured by a horizontal load cell between the tractor
and the towed implement. Vertical forces were ignored.
Ground Speed - the speed that a tractor was moving
was measured by a radar unit placed on the tractor and
aimed forward.
Wheel or Track Speed - the speed the surface of a
power delivery system was moving was measured by a
11
representing some 100 to 300 individual readings on
each data channel. Once a maximum for a gear was
reached, whether slip or engine load, additional data was
taken around the 10 percent slip level and around the
engine rated speed, when either or both points could be
reached.
A surface moisture sample and a subsoil moisture
sample were taken at most sites. The surface moisture
was determined as the dry basis average of the top 100
mm (4 in), or the tilled area of the soil, and the subsoil
sample by the next 100 mm (4 in).
radar unit placed on the tractor and aimed at the wheel or
track.
Vertical Acceleration - the acceleration or shaking of
a tractor along the up-down axis was measured by an
accelerometer placed near the centerline of the front axle.
Engine Power Level - the power produced by an
engine, was measured in several ways. On some
tractors, power levels were measured directly with a
torque meter and speed pick-up placed between the
engine and transmission. On others, power level
substitutes such as engine speed, turbo boost pressure
or fuel rail pressure were measured. When a substitute
signal was used, the signal was first calibrated to the
actual power level using the PTO substitute method (11).
Engine Fuel Consumption - the amount of fuel an
engine burned at a given power level was measured
during the 1991 tests only. On one tractor this was
measured directly with a fuel flow metering system. On
the others, fuel consumption substitutes such as engine
speed, turbo boost pressure or fuel rail pressure were
measured. When a substitute signal was used, the signal
was first calibrated to the actual fuel consumption using
a PTO load and a timed interval test.
The values that were calculated included the
following:
Slip - the amount of lost motion between the track or
wheel and the ground was computed from the signals of
the two radar guns (14).
Drawbar Power - the amount of power produced at
the rear of the tractor and delivered to an implement was
computed from the pull and the vehicle speed.
Percent Power Delivery - the percentage of the power
produced by the engine that was available at the rear of
the tractor and delivered to an implement was computed
by dividing the power produced at the rear by the power
produced at the PTO. This is a substitute or analog for
tractive efficiency that can be used in the same manner
as tractive efficiency (11), and was the main performance
comparison used in the tests.
Specific Fuel Consumption - the amount of fuel
burned to produce a unit of drawbar power was computed
by dividing the fuel used by the drawbar power produced.
Test Procedure - The test procedure for a traction
system was the AFMRC simplified procedure (11) that
determined the entire range of performance for a tractor.
A tractor was set to a specific tire, ballast and tire inflation
combination and the instrumentation was calibrated. The
tractor was operated in the field using a chisel plow as a
variable load and tested in at least three different gears.
One was a gear low enough to allow overloading the
traction system to produce excessive slip (40 to 50
percent). A second gear was in the normal operating
range. A third was a gear high enough to overload the
engine at low (5 to 10 percent) slip levels. In each of
these gears, the test was started with the implement out
of the ground. The draft was then increased from zero
up to the maximum in a series of small increments. Once
the tractor reached equilibrium after a draft increment, a
10 to 30 second data snapshot was recorded,
SITE SPECIFIC TEST NOTES
Alberta 1991 - Tests were completed in the low
organic clay-loam, sandy and heavy clay soil during July
and August. The Case-ill 9250 and 926.0 four-wheel
drive tractors were equipped with dual 20.8 R38 radial
tires. The Caterpillar Challenger 65 was equipped with
two 610 mm (24 in) wide rubber belt tracks. Prior to the
field tests, the PTO power produced, engine speed and
fuel consumed versus engine load were measured at the
AFMRC lab using a dynamometer and an AFMRCdeveloped fuel mass flow system. These numbers were
used to compute the power and fuel consumption .levels
from engine speed during field tests. The Case-IH 9250
pivot steer tractor did not have a PTO but was included
in the draft tests to observe any differences between a
pivot steer and a fixed frame four-wheel steer design. A
13 m (43 ft) Leon Manufacturing chisel plow was used for
the draft load at each site. All the tests were run with tire
inflation pressures of 83 kPa (12 psi), the correct and
recommended pressure for the tire load by tire
manufacturers in 1991. When compared to the revised
tire inflation guidelines released in 1992, the tires in these
tests were overinflated. The pressures could have been
reduced to 76 kPa (11 psi) front and 55 kPa (8 psi) rear.
Montana 1991 - Tests were conducted at one site, a
low organic heavy clay soil, in both primary and
secondary tillage, from mid-September to mid-October.
Four tractors were used - a John Deere 8760 and 8960
four-wheel-drive, a Caterpillar Challenger 75 with two 700
mm (27.5 in) wide tracks, and a Caterpillar Challenger 75
with two 890 mm (35 in) wide tracks. The John Deere
8760 was tested with single 24.5 R32 radial tires, dual
20.8 R42 radial tires and dual 20.8 42 bias-ply tires. The
John Deere 8960 was tested with dual 20.8 R42 radial
tires, triple 20.8 R42 radial tires, dual 710/70 R38 radial
tires, and single 710/70 R38 radial tires. As with previous
tractors, engine power and fuel consumption
measurements were made on both Deere tractors and on
the narrow-track Cat 75. Since the wide-track Cat did not
have a PTO, no power measurements were made on it.
The computer- controlled engine parameters were set by
the dealer to be identical on both machines. Both John
Deere tractors were equipped with engine torque meters
and readings from these were correlated with the
calculated values obtained from the PTO tests. Field
loads were provided by a 18 m (59 ft) farmer-modified
12
power level from engine speed during field tests. Field
loads were provided by a 13 m (43 ft) Friggstad chisel
plow. The tractor was run at only one ballast set-up.
John Deere chisel plow. The tractors were run with
various inflation pressure and ballast set-up combinations.
Alberta 1992 - Tests were completed in June in a
moderate organic loam soil, in August in a high organic
clay-loam soil, and in October in a low organic clay-loam
soil. A John Deere 8760 four-wheel drive with single,
dual and triple 20.8 R42 radial tires and a Challenger 65
with two 610 mm (24 in) wide tracks were tested in the
moderate organic loam soil. This condition was selected
as a condition where the flotation of the vehicles would be
a limit. The John Deere 8760 was tested in the high
organic clay-loam soil with dual 20.8 R42 radials along
with a Caterpillar Challenger 65B with 610 mm (24 in)
wide tracks. While this condition would normally have
been a very wet soil condition where flotation would be a
limit, during these tests the soil was unusually dry and
firm. The chisel plows used were 43 ft Flexi-coil units.
The John Deere 8760 was tested in the low organic
clay-loam soil with dual and triple 20.8 R42 radials. In
this test sequence, all the tests were run in the same strip
of the field. The strip was initially worked repeatedly to
produce a well tilled, loose soil condition and tests were
then repeated on the same strip over and over again.
The chisel plow used was a 14 m (45 ft) Morris. On the
Challenger 65 and the 65B, the PTO power was initially
measured with a PTO dynamometer and was then
calculated from engine speed during the field tests. The
John Deere 8760 was equipped with an engine torque
meter and engine power was measured directly during
the field tests and correlated with PTO and engine
readings from the tractor used in 1991. Fuel
measurements and calculations were not made in 1992.
The tractors were run with various inflation pressure and
ballast set-up combinations.
Alberta 1993 - Tests were completed in September in
a low organic clay-loam soil. A Ford New Holland 946
four-wheel drive was tested with single, dual and triple
20.8 R42 radial tires and a John Deere 8770 four-wheel
drive was tested with dual and triple 20.8 R42 radial tires.
Both tractors were equipped with in-line engine
dynamometers that measured engine power directly
during the field tests. The Ford New Holland was also
equipped with a rear driveline dynamometer. This
allowed the determination of the power split from the front
axle to rear axle. Fuel consumption measurements and
calculations were not made in 1993. As in the final test
sequence of 1992, all these tests were run in one well
worked strip of the field. The chisel plow used was a 14
m (45 ft) Flexi-coil. Various inflation pressure and ballast
set-up combinations were tested.
Alberta 1994 - Tests were completed in November in
both primary and secondary tillage in a low organic
clay-loam soil. A Case-IH 9250 four-wheel drive that had
been modified to be a four-track machine was tested.
The wheel assemblies had been replaced with four
separate positive drive rubber belt track assemblies.
Prior to the tests, the PTO power produced and the
engine speed were measured at the AFMRC lab using a
dynamometer. These numbers were used to compute the
FOUR TRACK MACHINE DESCRIPTION
The four track rubber belt tractor tested by the Alberta
Farm Machinery Research Centre was a Case-IH 9250
articulated four-wheel drive tractor modified into a four
track machine. The wheel assemblies had been replaced
with four separate triangular GripTrac (TM) rubber belt
track assemblies manufactured by the Gilbert and Riplo
Company of Ravenna, Michigan. The tractor and track
combination was custom-built by a local Case-IH dealer
to be similar to the QuadTrac (TM) prototypes displayed
and demonstrated at farm shows by Case Corporation.
A side view of the tractor is shown in Figure A1 and
front view in Figure A2.
Figure A1. Side View of Tractor.
Figure A2. Front View of Tractor.
13
The drive roller for each GripTrac (TM) assembly was
bolted to the tractor axle hub where the wheel rims had
been. Each drive roller was 990 mm (39 in) in diameter
and was constructed with a centre section divided with 20
rounded-edge plates that served as gear teeth. A
structural member ran across the tractor underneath each
axle and provided a pivot point for each track assembly
that was directly below the centre line of the drive roller.
Each track assembly formed an assymetrical triangle
about the drive roller. The track frame held five lower
wheels, a front and rear 500 mm (19.75 in) diameter idler,
and three 250 mm (10 in) diameter rollers. Four of the
rollers were rigidly mounted in the lower frame bar. Belt
tensioning was accomplished with a hydraulic
accumulator system that pushed one end idler (either
front or rear, depending on how the track was mounted)
out along the axis of the frame. The rubber belt tracks
were 775 mm (30.25 in) wide belts with a chevron-ground
engaging lug pattern on the outside surface and centre
drive lugs on the inside surface. Typically seven of the
inner lugs were fully engaged in the drive roller teeth.
Since the diameter of the drive roller was less than the
diameter of the original drive tires, the net gear ratio of
the tractor and, hence the speed in each gear, was
reduced by some 30 percent.
Figure A3 is a photo of a side view of a single track
and Figure A4 is a drawing of a track showing the
significant dimensions.
Figure A4. Track Assembly Dimensions.
Figure A5 gives dimensions for the overall tractor
configuration. As shown in Figures A1 and A5, on the
test tractor the rear track frames were reversed compared
to the front tracks. This gave extra clearance in the
centre of the tractor for turning, but it also meant that the
rear tracks ran with the tensioning idler on the tight side
of the rubber belt. The tractor and track assemblies had
been operated for some 200 hours and there was no
indication that this had been a problem.
Figure A5. Overall Tractor Dimensions.
The tractor was equipped with a PTO and a
three-point hitch. Total weight of the tractor with a threequarter full fuel tank was 16,700 kg (36,850 lb). The
static front axle weight was 9,200 kg (20,280 lb) or 55
percent of total weight, and the rear axle weight was
7,500 kg (16,570 lb), 45 percent of total weight. There
was no additional ballast on the tractor.
Figure A3. Side View of a Single Track.
14